Electric lamp with a dip-coated fired layer



Sept. 14, 1965 F. RoKosz 3,206,632

ELECTRIC LAMP WITH A DIP-COATED FIRED LAYER Filed April 21, 1961 FIG. 4.

PR EPARE ORGAN O- METALLIC LUSTRE SOLUTION ADMIX WITH ORGANIC LACQUER AND SOLVENT TO FORM LUSTRE COATING COMPOSITION.

DIP ARTICLE III COATING COMPOSITION.

FIG. 2. I

DRAIN COATED ARTICLE.

2 g AIR GR 4 .D FORCE DRY.

WAVE LENGTH- ANGSTROMS FIG. 3.

FIRE AT A PREDETERMINED TEMPERATURE TO DECOMPOSE LUSTRE COMPOUND AND DEPOSIT COLLOIDAL METAL 5 COATING.

Lu A

INV EN TOR. G E

I FERDINAND ROKOSZ. Bun-z GREEN RED WAVE LENGTH- ANGSTROMS AGENT.

United States Patent 3,206,632 ELECTRIC LAMP WITH A DIP-COATED FIRED LAYER Ferdinand Rolrosz, Clifton, N.J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, 21 corporation of Pennsylvania Filed Apr. 21, 1961, Ser. No. 104,606 6 (Jlaims. (Cl. 313-25) This invention relates to electric lamps and, more particularly, to an improved high pressure mercury vapor lamp.

High pressure mercury vapor (HPMV) lamps characteristically have a light output that is overly rich in green and yellow radiations and deficient in red radiation. This undesirable color imbalance and resultant poor color rendition has been corrected to a very large degree by the development of various UV-responsive redemitting high temperature phosphors which, when applied to the inner surface of the protective envelope that encloses the arc tube, supplement the weak red emission of the mercury arc discharge.

While the aforesaid fluorescent-mercury vapor lamps have a more balanced light output as regards color, the yellow and green components still predominate thereby preventing such lamps from being used in such specialized applications as office, store or school lighting etc., where even slight deviations from natural color rendition are objectionable.

It is accordingly the general object of this invention to provide a simple and inexpensive means for modifying the character of the light output of an electric lamp.

Another object is the provision of an HPMV lamp which can be conveniently and efiiciently fabricated on a production line basis and is suitable for use in applications that require good color rendition.

Still another object is the provision of a process for applying a filter coating of uniform density to an electric lamp or the like, either during or after the fabrication thereof.

The aforesaid objects, and others which will become apparent as the description proceeds, are achieved by dipping the envelope (or the bulb portion of the completed lamp) into a coating composition which, when fired, deposits a colloidal metal lustre of uniform density on the envelope that selectively filters out the undesired radiations. In the case of an I-IPMV lamp, the dip coating composition contains an organo-gold compound which decomposes during the firing operation to form a colloidal gold layer or film of preselected density that strongly absorbs yellow-green radiations and thereby constitutes an integral color-corrective filter for the lamp.

A better understanding of the invention will be obtained by referring to the accompanying drawing where- FIG. 1 .is a side elevational view on a reduced scale of an improved 400W fluorescent-mercury lamp manufactured in accordance with the present invention;

FIG. 2 is a graph illustrating the spectral transmittance of the color-corrective filter coating employed on the lamp shown in FIG. 1;

FIG. 3 is a graph comparing the spectral distribution curves of a typical fluorescent-mercury lamp and the same lamp provided with the aforesaid color-corrective filter coating; and

FIG. 4 is a block diagram illustrating the dip coating process for depositing the aforesaid filter coating on the lamp envelope in accordance with this invention.

While the dip coated metallic lustre film of the present invention may be employed to modify the light output of various types of lamps, such as incandescent lamps 3,266,632 Patented Sept. 14, 1965 for example where warm red-tinted light is desired, it is especially adapted for use in conjunction with fiuorescentmercury lamps and has accordingly been so illustrated and will be so described.

With specific reference now to the drawing, in FIG. 1 there is shown a fluorescent-mercury lamp 10 which generally comprises an outer or enclosing envelope 12 having an arc tube 14 sealed therein that is supported in coaxial relationship therewith by means of a suitable wire frame assembly 15 and a reentrant stem 16. The inner surface of the enclosing envelope 12 is coated with a layer 18 of suitable fluorescent material, such as manganese-activated magnesium fiuorogermanate phosphor or the like that emits mainly in the red region of the spectrum in response to impinging UV radiations generated by the arc tube. A base 20 is attached to the envelope neck in the usual manner. Fluorescent-mercury discharge lamps of this type are well-known in the art and a detailed description thereof is given in U.S. Patent No. 2,748,303 issued May 29, 6 to L. Thorington.

In the absence of high temperature phosphors that have a sufficiently strong red emission to counterbalance the predominate yellow and green components of the mercury arc discharge, attempts have been made to provide integral'subtractive color filters capable of improving the color balance of the aforesaid fluorescent-mercury vapor lamp. In view of the long-life advantage afforded by such lamps, it is economically feasible to sacrifice up to about 20 to 25% of the light output by decreasing the yellow-green emission so that the additional improvement in color rendition required for such specialized applications as store and school lighting etc. can be obtained. In the case of mercury vapor lamps it was found that a purple-tinted colloidal gold lustre or coating of the proper density was very elfective as a color-corrective filter. As shown in FIG. 2, such a coating on a glass substrate strongly absorbs radiations in the yellow-green region of the spectrum but has relatively good transmittance in the other regions, particularly in the red.

Various so-called lustre solutions are commercially available which can be employed to deposit a colloidal layer of a noble metal on glass or the like. These lustre solutions generally consist of a volatile organic diluent, such as nitrobenzene or chloroform or the like or mixtures thereof, and an organo-metallic compound or resinate which decomposes when heated to form the desired colloidal metal lustre or coating. The organo-metallic compound can comprise an organic compound of the salt of the particular metal, as is disclosed in U.S. Patent No. 2,398,712. Organo-gold compounds and the deposition of colloidal gold coatings by this technique per se are well known, see for example The Industrial Chemist, July 1960, pp. 325330 and the aforesaid U.S. patent. Various types of such compounds are also described in The Chemical Elements and Their Compounds, N. V. Sidgwick, 1950, vol. I, pp. 177-192 (Oxford University Press).

Heretofore, it has been the standard practice to ad- IIllX the organo-gold compound with a quick-drying organic vehicle and a suitable solvent and to apply the resulting lustre solution by brushing or spraying it onto the lamp envelope 12. In view of the inherent sensitivity of such purple gold lustres to lint, dust and variations in the coating thickness and density caused by the configuration of or surface imperfections in the envelope or article being coated, it was the general belief in both the lamp and coating industries that the only practical way to apply the lustre was to brush or spray the solution onto the article or substrate.

However, in addition to being an awkward time-consuming operation, brush application was found to be very impractical as regards the mass production of HPMV lamps because of the great variations which inevitably occurred in the density and filtering properties of the finished coating. While spray application was relatively less time consuming, it nevertheless also presented serious problems in that it was very difiicult to obtain coatings of the required uniform thickness and density because of the inability to maintain the proper relationship between such variables as the spray velocity, scanning speed, etc. As a result, lamps with spray coatings inherently had a mottled or unsightly non-uniform appearance and were generally unsatisfactory.

In addition, it was inevitable that a considerable portion of the coating material was wasted because of overspray resulting in a proportionate increase in the manufacturing cost per lamp, especially in the instant application where an expensive gold lustre is involved.

All of the foregoing problems and difficulties are overcome in accordance with the present invention by providing the fluorescent-mercury lamp with a dip-coated colloidal gold film or layer 22 that is not only very uniform and of the proper density but is very simple and inexpensive to apply, even on a mass production basis.

As shown in FIG. 4, the dip coating composition is formed by admixing a suitable organo-metallic lustre solution with preselected amounts of a suitable organic lacquer and a solvent to form a lustre coating composition having the proper viscosity. In the particular case here illustrated, a commercially available organo-gold lustre solution, such as rose-purple lustre #1033 marketed by the Hanovia Liquid Gold Division, Engelhard Industries, Inc., East Newark, New Jersey, was mixed with an organic film-forming lacquer or vehicle such as ethylcellulose and a volatile solvent such as toluene to form a lustre coating composition having a viscosity of about '60 centipoises at about 25 C., as measured with a Model RVF Brookfield viscometer using the #2 spindle at 30 rpm. Of course, the viscosity will vary somewhat de pending upon the particular type of lacquer used.

A specific example of a suitable lustre coating composition is as follows:

Example I Constituent: Amount Rose-purple lustre #1033, (Engelhard Industries, Inc.) cc 60 Toluene cc 2000 Ethylceilulose grams 35 Other examples of suitable organic film-forming lacquers or vehicles to which the aforesaid purple lustre or other solution can be added to form dip-coating lustre compositions of the proper viscosity are as follows:

Other organic film-forming lacquers or materials such as cellulose acetate butyrate, cellulose acetate propionate and vinyl acetate with appropriate solvents can also be used.

After the aforesaid lustre coating composition has been prepared, the outer or enclosing envelope 12 is cleaned to remove any surface dust or dirt, dipped into the coating composition and quickly withdrawn and inverted into a neck-down position. Either the envelope per se or the finished lamp may be coated in this manner.

The coated envelopes are then allowed to drip-drain so as to permit leveling of the coating to occur. They are then air dried, or force dried if desired in an oven for approximately ten minutes at to C., to volatilize the solvents and set the coating. The coated envelopes or lamps are then placed into an oven and baked in an air atmosphere atabout 600 C. for approximately ten minutes to fire the coating and thereby decompose the organo-gold compound and the organic film-forming vehicle and deposit a colloidal gold layer 22 of preselected density on the envelope surface. If necessary, the firing temperature may be reduced to about 525 C. to avoid deforming the envelope and the firing time proportionately increased.

It has been found that dip-coated color-corrective colloidal gold films or lustres formed in the aforesaid manner not only tenaciously adhere to the glass but are smooth and of very uniform density and thickness. Moreover, the density of the finished coating can be very accurately controlled simply by controlling the amount of the lustre solution that is added to the film-forming lacquer and by properly adjusting the viscosity of the latter. The waste and difiicult control problems associated with brush and spray coatings are entirely eliminated thereby enabling envelopes of any shape or size to be quickly coated on a mass production basis with a minimum amount of labor and material.

Because of the more uniform coating density obtained with the aforesaid dip coating process, it has been found that the red lumen output of a dip-coated fluorescentmercury vapor lamp is increased in the average by about 1% as compared to an identical lamp that has been spray coated resulting in a corresponding improvement in color balance and color rendition.

While the aforesaid lustre coating compositions may be varied somewhat, drastic changes either in the combination of materials or concentrations thereof cannot be tolerated insofar as the flow characteristics of the coating composition will be materially altered resulting in a finished coating that is either too thick or too thin to effect the precise degree of filtering required.

As shown in FIG. 3, the uniform colloidal gold filter coating 22 produced by the instant dip coating process modifies the spectral distribution of a typical fluorescentmercury lamp by reducing the intensity of the yellow and green components by about 20%, as is indicated by the shaded portions A which represents the difference in output of the two lamps. The outputs in the 6000-7200A region have been made equal to one another so that the relative output at the other wavelengths can be directly compared. While the color-corrective coating 22 also reduces somewhat the violet emission of the lamp, as indicated by the shaded portions B of the curves shown in FIG. 3, this has very little effect either on the lamp efficiency or overall color-balance of the light output.

It will be apparent from the foregoing that the objects of the invention have been achieved by providing a fluorescent-mercury lamp that has an improved output as regards color balance and rendition and which can be provided with a very uniform color-corrective filter coating of preselected density that is very convenient and inexpensive to apply.

While one specific lamp embodiment has been illustrated and described, it will be appreciated that other types of lamps can be dip-coated and various modifications can be made in the dip-coating compositions without departing from the spirit and scope of this invention. For example, a yellow lustre coating may be applied to a highpressure gaseous discharge lamp in accordance with the teachings of this invention to provide a lamp that has a yellow light output and is useful in providing caution lighting at highway intersections, etc.

I claim:

1. The process of coating an electric lamp envelope or the like with a colloidal-metal film of predetermined and uniform thickness that selectively absorb-s radiations of a preselected wave length, which process comprises,

(1) preparing a coating composition that has a predetermined viscosity and consists essentially of (a) a liquid film-forming organic vehicle selected from the group consisting of nitrocellulose, ethylcellulose, acrylic resin, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate and vinyl acetate; (1b) a lustre solution containing an organic compound of the metal dispersed in said film-forming vehicle; and (c) a volatile solvent selected from the group consisting of toluene, Xylene, ethyl alcohol, ethyl acetate, acetone, ethanol and mixtures thereof;

(2) dipping the lamp envelope into said coating composition;

(3) and then heating the coated envelope to a predetermined temperature sufiicient to decompose the organo-metallic compound and film-forming vehicle and thereby deposit a tenaciously adhering and uniform layer of colloidal metal on said envelope.

2. An electric lamp comprising, a light-transmitting vitreous envelope, a light source sealed within said envelope, and means for modifying the color of the light normally produced by said lamp comprising a dip-coated fired layer of colloidal metal on said envelope that absorbs radiation of a preselected Wavelength generated by said light source 3. The electric lamp set forth in claim 2 wherein said color-modifying means comprises a dip-coated fired layer of colloidal noble metal of preselected and uniform density.

4. In combination with a high-pressure mercury vapor discharge lamp having an arc tube and an enclosing envelope of vitreous light-transmitting material, means for selectively absorbing visible radiations generated by said arc tube and thereby modifying the color of the light normally produced by said lamp comprising a dip-coated fired layer of colloidal metal on the outer surface of said envelope.

5. The combination set forth in claim 4 wherein said color-modifying means comprises a dip-coated fired layer of colloidal gold that absorbs primarily yellow-green radiations.

6. The combination set forth in claim 4 wherein the inner surface of said enclosing envelope is coated with a fluorescent material that emits radiation mainly in the red region of the spectrum in response to ultraviolet radiation generated by said are tube, and said color-modifying means comprises a dip-coated fired layer of colloidal gold of preselected and uniform density that absorbs yellowgreen radiations.

References Cited by the Examiner UNITED STATES PATENTS 2,966,605 12/60 Harris et a1 3131 12 X GEORGE N. WESTBY, Primary Examiner.

GAUSS, Examiner. 

4. IN COMBINATION WITH A HIGH-PRESSURE MERCURY VAPOR DISCHARGE LAMP HAVING AN ARC TUBE AND AN ENCLOSING ENVELOPE OF VITREOUS LIGHT-TRANSMITTING MATERIAL, MEANS FOR SELECTIVELY ABSORBING VISIBLE RADIATIONS GENERATED BY SAID ARC TUBE AND THEREBY MODIFYING THE COLOR OF THE LIGHT NORMALLY PRODUCED BY SAID LAMP COMPRISING A DIP-COATED FIRED LAYER OF COLLOIDAL METAL ON THE OUTER SURFACE OF SAID ENVELOPE. 